Progress in underwater acoustic telemetry since 1982 is reviewed within a framework of six current research areas: (1) underwater channel physics, channel simulations, and measurements; (2) receiver structures; (3) diversity exploitation; (4) error control coding; (5) networked systems; and (6) alternative modulation strategies. Advances in each of these areas as well as perspectives on the future challenges facing them are presented. A primary thesis of this paper is that increased integration of high-fidelity channel models into ongoing underwater telemetry research is needed if the performance envelope (defined in terms of range, rate, and channel complexity) of underwater modems is to expand.

/pdf/the-state-of-the-art-in-underwater-acoustic-telemetry-25guddsizz.pdf

The state of the art in underwater acoustic telemetry

https://inspirehep.net/files/d93a99e6425dfc65d02f62e0394a907c

Factorization and asymptotic behaviour of pion form factor in QCD

Asymptotic behaviour of exclusive processes in QCD

https://cds.cern.ch/record/198545/files/198908347.pdf

An investigation of the spin structure of the proton in deep inelastic scattering of polarised muons on polarised protons

Analytical expressions for three P-wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework. Two of the models concern wave-induced flow due to heterogeneity in the elastic moduli at mesoscopic scales (scales greater than grain sizes but smaller than wavelengths). In the first model, the heterogeneity is due to lithological variations (e.g., mixtures of sands and clays) with a single fluid saturating all the pores. In the second model, a single uniform lithology is saturated in mesoscopic ''patches'' by two immiscible fluids (e.g., air and water). In the third model, the heterogeneity is at ''microscopic'' grain scales (broken grain contacts and/or micro-cracks in the grains) and the associated fluid response corresponds to ''squirt flow''. The model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus, crack porosity, and/or frequency is reduced to zero. It is shown that squirt flow is incapable of explaining the measured level of loss (10{sup -2} < Q{sup -1} < 10{sup -1}) within the seismic band of frequencies (1 to 10{sup 4} Hz); however, either of the two mesoscopic scale models easily produce enough attenuation to explain the field data.

/pdf/seismic-attenuation-due-to-wave-induced-flow-4pg4hfkmld.pdf

Seismic attenuation due to wave-induced flow

The validity of the perturbation approximation for rough surface scattering is examined (1) by comparison with exact results obtained by solving an integral equation and (2) through comparison of low‐order perturbation predictions with higher‐order predictions. The pressure release boundary condition is assumed, and the field quantity calculated is the bistatic scattering cross section. A Gaussian roughness spectrum is used, and the surfaces have height variations in only one direction. It is found, in general, that the condition kh≪1 (k is the acoustic wavenumber, h is the rms surface height) is insufficient to guarantee the accuracy of first‐order (or higher‐order) perturbation theory. When the surface correlation length l becomes too large or too small with h held fixed, higher‐order perturbation terms can make larger contributions to the scattering cross section than lower‐order terms. An explanation for this result is given. The regions of validity for low‐order perturbation theory are also given. Th...

The validity of the perturbation approximation for rough surface scattering using a Gaussian roughness spectrum

An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time-reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source–receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. Phase conjugation was implemented by transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal and retransmitting the time reversed signals from all the sources of the SRA. The retransmitted signal then was received at the VRA. An assortment of runs was made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable, and the experimental results were consistent with theory.

Phase conjugation in the ocean: Experimental demonstration of an acoustic time-reversal mirror

Phase‐conjugate mirrors are used in optics to compensate for aberrations caused by inhomogeneities in the propagation medium and by imperfections in optical components. In acoustics, analogous behavior can be achieved by a time‐reversed retransmission of signals received by an array. Compensation for multipath propagation and array imperfections is automatic and does not require knowledge of the detailed properties of either the medium or the array. The behavior of acoustic phase‐conjugate arrays is illustrated in several examples, some highly idealized and some more realistic. The effects of aperture size and inhomogeneities in the propagation medium are treated for both the near‐field and far‐field regions. It is concluded that phase‐conjugate arrays offer an attractive approach to some long‐standing problems in underwater acoustics.

Phase conjugation in underwater acoustics

The composite roughness model is applied to bottom backscattering in the frequency range 10–100 kHz. For angles near normal incidence, the composite roughness model is replaced by the Kirchhoff approximation which gives better results. In addition, sediment volume scattering is treated, with account taken of refraction and reflection at the randomly sloping interface. In applying the model to published data it is found that sediment volume scattering is dominant in soft sediments except at small and large grazing angles. For coarse sand bottoms, roughness scattering dominates over a wide range of grazing angles. Implications for acoustic remote sensing are discussed.

Application of the composite roughness model to high‐frequency bottom backscattering

In a colored-quark and vector-gluon model of hadrons we show that a quark carrying nearly all the momentum of a nucleon (x≈1) must have the same helicity as the nucleon; consequently νW^2_n/νW^p_2→(3/7) as x→1, not (2/3) as might naively have been expected. Furthermore as x→1, νW^(π)_2∼(1-x)^2 and (σ_(L)/σ_(T))π∼μ^(2)Q^(-2)(1-x)^(-2)+O(g^2); the resulting angular dependence for e^(+)e^(-)→h^(±)+ X is consistent with present data and has a distinctive form which can be easily tested when better data are available.

/pdf/pion-and-nucleon-structure-functions-near-x-1-2jplsqedve.pdf

Darrell R. Jackson

Papers

The validity of the perturbation approximation for rough surface scattering using a Gaussian roughness spectrum

Phase conjugation in the ocean: Experimental demonstration of an acoustic time-reversal mirror

Phase conjugation in underwater acoustics

Application of the composite roughness model to high‐frequency bottom backscattering

Pion and Nucleon Structure Functions near x = 1